Slow Shock Formation Upstream of Reconnecting Current Sheets

TL;DR

This paper investigates the formation and role of slow shocks upstream of reconnecting current sheets in magnetic reconnection, highlighting their contribution to plasma heating and their dynamics in multi-island reconnection scenarios.

Contribution

It provides detailed analysis of slow shock formation in kinetic and MHD simulations, emphasizing their role in plasma preheating and the dynamics of multi-island reconnection.

Findings

Slow shocks form upstream during multi-island reconnection.

Shocks satisfy Rankine-Hugoniot conditions and contribute to plasma heating.

Their impact on electron heating is minor compared to other mechanisms.

Abstract

The formation, development and impact of slow shocks in the upstream region of reconnecting current layers are explored. Slow shocks have been documented in the upstream region of magnetohydrodynamic (MHD) simulations of magnetic reconnection as well as in similar simulations with the {\it kglobal} kinetic macroscale simulation model. They are therefore a candidate mechanism for preheating the plasma that is injected into the current layers that facilitate magnetic energy release in solar flares. Of particular interest is their potential role in producing the hot thermal component of electrons in flares. During multi-island reconnection, the formation and merging of flux ropes in the reconnecting current layer drives plasma flows and pressure disturbances in the upstream region. These pressure disturbances steepen into slow shocks that propagate along the reconnecting component of the magnetic field and satisfy the expected Rankine-Hugoniot jump conditions. Plasma heating arises from both compression across the shock and the parallel electric field that develops to maintain charge neutrality in a kinetic system. Shocks are weaker at lower plasma $\beta $, where shock steepening is slow. While these upstream slow shocks are intrinsic to the dynamics of multi-island reconnection, their contribution to electron heating remains relatively minor compared with that from Fermi reflection and the parallel electric fields that bound the reconnection outflow.